1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an array substrate for an in-plane switching (IPS) liquid crystal display device and a method of fabricating the same.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That alignment direction can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.
Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin and have low power consumption characteristics. The typical liquid crystal display panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
In a conventional LCD device, since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. However, the conventional LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. IPS-LCD devices include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation of the operation modes of a related art IPS-LCD panel will be provided referring to FIG. 1.
FIG. 1 is a schematic cross-sectional view illustrating a concept of a related art IPS-LCD panel. As shown in FIG. 1, first and second substrates 10 and 20 are spaced apart from each other, and a liquid crystal layer 30 is interposed therebetween. The first and second substrates 10 and 20 are often referred to as an array substrate and a color filter substrate, respectively. On the first substrate 10 are a first electrode 12 and a second electrode 14. The first and second electrodes 12 and 14 are aligned parallel to each other. The first electrode 12 may function as a common electrode and the second electrode 14 may serve as a pixel electrode. On a surface of the second substrate 20, a color filter layer (not shown) is commonly positioned between the second electrode 14 and the first electrode 12 of the first substrate 10. A voltage applied across the first and second electrodes 12 and 14 produces an electric field 16 through liquid crystal molecules 32 of the liquid crystal layer 30. The liquid crystal layer 30 has a positive dielectric anisotropy, and thus the liquid crystal molecules 32 align parallel to the electric field 16.
When no electric field is produced by the first and second electrodes 12 and 14, i.e., off state, the longitudinal axes of the liquid crystal (LC) molecules 32 are parallel and form a definite angle with the first and second electrodes 12 and 14. For example, the longitudinal axes of the LC molecules 32 are arranged parallel with both the first and second electrodes 12 and 14.
On the contrary, when a voltage is applied to the first and second electrodes 12 and 14, i.e., on state, an in-plane electric field 16 that is parallel to the surface of the first substrate 10 is produced because the first and second electrodes 12 and 14 are on the first substrate 10. Accordingly, the LC molecules 32 are re-arranged to bring their longitudinal axes into coincidence with the electric field 16.
Therefore, the result is a wide viewing angle that ranges from about 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD panel, for example.
FIG. 2 is a plan view illustrating one pixel of an array substrate for an IPS-LCD device according to the related art. As shown in FIG. 2, a gate line 52 is formed in a first direction and a data line 54 is formed in a second direction crossing the gate line. The gate and data lines 52 and 54 define a pixel region P on the array substrate. A thin film transistor T is formed at a crossing of the gate and data lines 52 and 54. In the pixel region P, a first connecting line 56 connected to the thin film transistor T is formed in the first direction, and a plurality of pixel electrodes 58 extends from the first connecting line 56 along the second direction. The plurality of pixel electrodes 58 are spaced apart from each other, and one end of the plurality of pixel electrodes 58 is connected to a second connecting line 60. A common line 62 is formed parallel to the gate line 52. The common line 62  crosses centers of the plurality of pixel electrodes 58. A plurality of common electrodes 64 extends from the common line 62 upward and downward in the context of the figure. The plurality of common electrodes 64 is alternatively arranged with the plurality of pixel electrodes 58.
To remove a noise field between the data line 54 and the pixel electrodes 58, which causes poor images, the common electrode 64 lies close by the data line 54.
For example, the common line 62 and the common electrode 64 are formed of the same material through the same process as the gate line 52. The first and second connecting lines 56 and 60 and the pixel electrodes 58 are formed of the same material through the same process as the data line 54. The thin film transistor T includes a gate electrode 66 that extends from the gate line 52, a source electrode 68 that extends from the data line 54, a drain electrode 70 that is spaced apart from the source electrode 68, and a semiconductor layer 72 that covers the gate electrode 66 and overlaps the source electrode 68 and the drain electrode 70 in part. An intrinsic material of the semiconductor layer 72 is selected from amorphous silicon. The thin film transistor T may be an inverted staggered type. The drain electrode 70 may be formed integral with the first connecting line 56.
In the IPS-LCD device, a region in which liquid crystal molecules are driven corresponds to an area in which the liquid crystal molecules are horizontally arranged by a lateral electric field. That is, the region includes spaces between the common electrode 64 and the pixel electrode 58 and sides of the common electrode 64 and the pixel electrode 58. However, in the above structure, since the common and pixel electrodes are made of an opaque metal material, images are displayed only in the spaces between the common and pixel electrodes 64 and 58 to lower an aperture ratio.
To improve the aperture ratio, one of the common and pixel electrodes may be formed of a transparent conducting material.
FIG. 3 is a plan view illustrating one pixel of an array substrate for an IPS-LCD device having an improved aperture ratio according to the related art. The array substrate of FIG. 3 may have the same structure as that of FIG. 2, and some of the detailed explanations, previously explained with reference to FIG. 2 will be omitted in order to prevent duplicate explanations.
As shown in FIG. 3, a thin film transistor T, which includes a gate electrode 82, a semiconductor layer 84, a source electrode 86 and a drain electrode 88, is formed at a crossing of a gate line 72 and a data line 74. The drain electrode 88 is connected to a first connecting line 92 through a drain contact hole 90. A plurality of pixel electrodes 94 extends from the first connecting line 92, and one end of the pixel electrodes 94 is connected to a second connecting line 96. The first and second connecting lines 92 and 96 and the pixel electrodes 94 may be made of a transparent conducting material, beneficially indium-tin-oxide (ITO).
A common line 98 and a plurality of common electrodes 99 may have the same structures and may be made of the same material as those of FIG. 2.
Selection of the transparent conducting material as an electrode material for improving the aperture ratio may have the following problems.
First, the transparent conducting material may be patterned by a wet etching method like the opaque metal material. A critical dimension (CD), which may be defined as a design rule of a pattern in the panel, may be non-uniform in each region due to the wet etching process. In addition, since the transparent conducting material transmits light, scratches on a back surface of the substrate by a chuck for supporting the substrate may have large effects on an exposing process of the substrate, whereby patterns having uneven sides may be formed. Therefore, spots in the images may occur, and image qualities may be lowered.
Second, because the drain contact hole may be necessary to connect the electrode of the transparent conducting material with the metal line, the number of manufacturing processes may be increased. If the common and pixel electrodes are formed of the transparent conducting material, more processes for forming contact holes may be needed, and thus problems may be increased.